1. Field of the Invention
The present invention relates generally to power measurement devices for radiant energy, particularly to power measurement devices and methods for use with coherent energy at relatively high power levels.
2. Related Art
Measurement of radiant energy has reached its most sophisticated and advanced levels with reference to laser power measurement. Typically such devices involve an absorber, which may be a flat plate absorbent at the wave length of the coherent radiation, or at somewhat higher power levels or more dense beams, a glass plate to provide a volume absorption of the laser beam. In the event that the energy density is such as to damage flat plate absorbers, either surface or volume types, it has been proposed to distribute the energy over an even greater surface, such as by obliquely directing the energy beam into a tube of an internal finish which only partially absorbs the energy such as to reflect, bounce and scatter the energy down the tube and diffuse the energy being absorbed at any given portion of the tube.
Typically, the energy is determined by conducting the absorbed energy in the form of sensible heat to a heat sink, air cooled in the case of relatively low power levels, and water cooled at higher power levels. The energy is thus measured by flowing the sensible heat through the energy measuring means. Such means include thermopiles, which are desirable for the zero heat flow characteristics, or other temperature sensing devices positioned between the absorber and the heat sink. Given the nature of the heat measuring device across or through which sensible heat energy flow is measured, it must be recognized that the heat measuring device retards heat flow from the absorber to the heat sink. This, in the case of high energy levels, causes the absorber to become heated and reradiate energy which is lost to the measurement. Also, great amounts of energy must flow through the heat measuring device. At relatively high energy levels, it has been found that flowing great amounts of sensible heat through the heat energy measurement device is quite troublesome in that, for instant in the case of the desirable thermopile arrangement, great numbers of thermocouples must be provided at substantial expense to accomodate the high energy flow, which may be in the order of thousands of watts. Also, when in direct contact with the absorber as is conventional, the heat flow through the temperature difference measuring means is not necessarily evenly distributed across relatively large areas of thermopiles, and flow through limited areas may be destructive to the measuring devices. Accordingly, it has been a usual practice to use less desirable components, i.e., a pair of independent temperature sensing devices with possibly non-identical characteristics, and to operate the absorber at elevated temperatures with the possibility of energy re-emission compromising the accuracy of the measurement.
In instances of devices with difficiency requirements and purposes, cooling fluid temperatures at the inlet and outlet portions have been measured to determine energy loss. However, this concept does not address or suggest the requirements and advantages of a cool absorber when measuring radiant energy.